Projection zoom lens and image projection apparatus

Abstract

A projection zoom lens includes, sequentially from an enlargement conjugate side: a first lens group having negative refractive power; a second lens group having positive or negative refractive power; a third lens group having positive refractive power; a fourth lens group having negative refractive power; a fifth lens group having positive refractive power; and a sixth lens group having positive refractive power, and configured to perform a magnification variation by changing the interval between the respective lens groups, wherein the fourth lens group includes at least one positive lens and at least one negative lens, at least one of the positive lenses included in the fourth lens group satisfies the following Conditional Expressions: 0.71≤θg_Fp+0.004×νdp≤0.73 and 15≤νdp≤25, and the back focus of the entire projection zoom lenses satisfies the following Conditional Expression 1.0≤BF/Fw≤2.7.

What is claimed is: 
     
       1. A projection zoom lens comprising, sequentially from an enlargement conjugate side: a first lens group having negative refractive power; a second lens group having positive or negative refractive power; a third lens group having positive refractive power; a fourth lens group having negative refractive power; a fifth lens group having positive refractive power; and a sixth lens group having positive refractive power, and configured to perform a magnification variation by changing the interval between the respective lens groups,
 wherein the fourth lens group comprises at least one positive lens and at least one negative lens, at least one of the positive lenses included in the fourth lens group satisfies the following Conditional Expressions (1) and (2), and the back focus of the entire projection zoom lenses satisfies the following Conditional Expression (3):
   0.71≤θ g _ Fp+ 0.004×ν dp≤ 0.73  (1)
 
   15≤ν dp≤ 25  (2)
 
   1.0≤ BF/Fw≤ 2.7  (3)
 
 
 where 
 θg_Fp: partial dispersion ratio of lens material, 
 θg_Fp=(Ng−NF)/(NF−NC) 
 Ng: refractive index of g line, 
 NF: refractive index of F line, 
 NC: refractive index of C line, 
 νdp: Abbe number of the lens material, 
 BF: air conversion back focus from the last lens surface to the image surface, and 
 Fw: focal length of the entire projection zoom lenses at the wide-angle end. 
 
     
     
       2. The projection zoom lens according to  claim 1 , wherein an aperture stop is disposed between a surface of the third lens group closest to a reduction conjugate side and a surface of the fifth lens group closest to an enlargement conjugate side. 
     
     
       3. The projection zoom lens according to  claim 1 , wherein the fourth lens group comprises two negative lenses and one positive lens sequentially from the enlargement conjugate side. 
     
     
       4. The projection zoom lens according to  claim 1 , wherein the fourth lens group comprises one negative lens and one positive lens sequentially from the enlargement conjugate side. 
     
     
       5. The projection zoom lens according to  claim 1 , wherein the positive lens, which satisfies Conditional Expressions (1) and (2), satisfies the following Conditional Expression (4):
   0.85≤|Φ p/Φ 4|≤1.45  (4)
 
 where 
 Φp: refractive power of a positive lens, and 
 Φ4: refractive power of the fourth lens group. 
 
     
     
       6. The projection zoom lens according to  claim 1 , wherein the following Conditional Expression (5) is satisfied:
   0.18≤|Φ4/Φ w|≤ 0.24  (5)
 
 where 
 Φ4: refractive power of the fourth lens group, and 
 Φw: refractive power of the entire projection zoom lens at the wide-angle end. 
 
     
     
       7. The projection zoom lens according to  claim 1 , wherein at least one of the negative lenses included in the fourth lens group satisfies the following Conditional Expressions (6) and (7):
   0.645≤θ g _ Fn+ 0.001682×ν dn≤ 0.695  (6)
 
   60≤ν dn≤ 100  (7)
 
 where 
 θg_Fn: partial dispersion ratio of the lens material, 
 θg_Fn=(Ng−NF)/(NF−NC) 
 Ng: refractive index of g line, 
 NF: refractive index of F line, 
 NC: refractive index of C line, and 
 νdn: Abbe number of the lens material. 
 
     
     
       8. An image projection apparatus comprising: an image forming element configured to form an image light; and the projection zoom lens according to  claim 1  configured to enlarge and project the image light.